Scandium is extremely useful as an additive for a high strength alloy and an electrode material for a fuel cell. However, scandium has not yet been used widely due to the small production quantity and high cost thereof.
Note that scandium is contained in nickel oxide ore such as laterite ore and limonite ore in a trace amount. However, nickel oxide ore has not been industrially used as a nickel raw material for a long due to the low grade of nickel contained. Hence, it has also been scarcely studied to industrially recover scandium from nickel oxide ore.
However, in recent years, the HPAL process has been emerging as a practical method, in which nickel oxide ore is introduced into a pressure vessel along with sulfuric acid and heated at a high temperature of about 240° C. to 260° C. to separate the mixture into a leachate containing nickel and a leach residue by solid-liquid separation. In this HPAL process, a neutralizing agent is added to the leachate obtained to separate impurities and then a sulfurizing agent is added to the leachate from which impurities have been separated to recover nickel as nickel sulfide. Thereafter, electric nickel or a nickel salt compound can be obtained by treating this nickel sulfide by a known nickel refinement process.
In the case of using the HPAL process as described above, scandium contained in nickel oxide ore is contained in the leachate along with nickel (see Patent Document 1). Thereafter, a neutralizing agent is added to the leachate obtained by the HPAL process to separate impurities and then a sulfurizing agent is added to the resulting leachate to recover nickel as nickel sulfide as well as to contain scandium in the acidic solution after the addition of a sulfurizing agent. Hence, it is possible to effectively separate nickel and scandium from each other by using the HPAL process.
In addition, there is also a method in which scandium is separated using a chelating resin (see Patent Document 2). Specifically, in the method disclosed in Patent Document 2, first, nickel and scandium are selectively leached from nickel-containing oxide ore into an aqueous acidic solution at a high temperature and a high pressure in an oxidizing atmosphere to obtain an acidic solution, subsequently the pH of the acidic solution is adjusted to a range of 2 to 4, and then nickel is selectively precipitated and recovered as a sulfide by use of a sulfurizing agent. Next, scandium is adsorbed to a chelating resin by bringing the solution obtained after nickel recovery into contact with the chelating resin, the chelating resin is washed with a dilute acid, and then scandium is eluted from the chelating resin by bringing the chelating resin after washing into contact with a strong acid.
In addition, as a method for recovering scandium from the acidic solution described above, a method for recovering scandium by solvent extraction has also been proposed (see Patent Documents 3 and 4). Specifically, in the method disclosed in Patent Document 3, first, an organic solvent prepared by diluting 2-ethylhexylsulfonic acid-mono-2-ethylhexyl with kerosene is added to a scandium-containing solution of an aqueous phase containing at least one or more kinds among iron, aluminum, calcium, yttrium, manganese, chromium, and magnesium in addition to scandium and the scandium component is extracted into the organic solvent. Subsequently, in order to separate yttrium, iron, manganese, chromium, magnesium, aluminum, and calcium which have been extracted into the organic solvent along with scandium, an aqueous solution of hydrochloric acid is added to the organic solvent and scrubbing is performed to remove these components, then an aqueous solution of NaOH is added to the organic solvent to obtain a slurry containing Sc(OH)3 converted from scandium remained in the organic solvent, Sc(OH)3 obtained by filtration of this slurry is dissolved with hydrochloric acid to obtain an aqueous solution of scandium chloride. Thereafter, oxalic acid is added to the aqueous solution of scandium chloride thus obtained to precipitate scandium oxalate, the precipitate is filtered to separate iron, manganese, chromium, magnesium, aluminum, and calcium into the filtrate, and then the precipitate filtered is calcined to obtain high purity scandium oxide.
In addition, Patent Document 4 describes a method for selectively separating and recovering scandium from a scandium-containing feed liquid by bringing a scandium-containing feed liquid into contact with an extractant at a fixed proportion by a batch treatment.
As the grade of scandium to be recovered by these methods, it is known to obtain a purity of about 95% to 98% in terms of scandium oxide. This grade is sufficient for applications such as addition to alloys, but a still higher purity, for example, a grade of about 99.9% is required in order to exert favorable properties for applications such as an electrolyte of a fuel cell which is increasingly demanded in recent years.
However, the kind and quantity of nickel oxide ore described above vary depending on the area from which the nickel oxide ore is produced and various impurity elements such as manganese and magnesium are contained in the nickel oxide ore in addition to iron and aluminum.
In the case of using scandium in applications such as an electrolyte of a fuel cell, the grade has an acceptable upper limit depending on the elements of impurities, and it is required to separate and remove the individual elements until the acceptable limit or less.
However, in the chelating resin and the organic solvent which have been disclosed in Patent Document 2 and Patent Document 3, several impurity elements exhibit behavior similar to scandium and it is thus difficult to effectively separate and recover scandium. In addition, the concentration of impurities such as iron and aluminum contained in the leachate of nickel oxide ore is still higher than that of scandium, these impurities in large amounts have also affected the recovery of scandium, and a suitable method for industrially recovering high purity scandium from nickel oxide ore has not been found out.
As described above, it has been difficult to efficiently separate a wide variety of impurities such as iron and aluminum contained in large amounts and to efficiently recover high purity scandium even when it is attempted to recover scandium from nickel oxide ore.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. H03-173725    Patent Document 2: Japanese Unexamined Patent Application, Publication No. H09-194211    Patent Document 3: Japanese Unexamined Patent Application, Publication No. H09-291320    Patent Document 4: PCT International Publication No. WO2014/110216